Acianthus, commonly known as mosquito orchids, is a genus of approximately twelve species of terrestrial orchids in the family Orchidaceae, order Asparagales. The genus was first formally described by the botanist Robert Brown in 1810 in his Prodromus Florae Novae Hollandiae, in which he named three founding species: A. fornicatus, A. exsertus, and A. caudatus. Mosquito orchids are closely related to the genus Caladenia.
These are small, perennial, deciduous, sympodial herbs. Each plant produces a single egg-shaped or heart-shaped basal leaf that is often purplish-red on its underside and may hug the ground or be held slightly above it. Rather than true roots, the plants have root-like stolons that terminate in small, roughly spherical underground tubers; new "daughter" tubers develop at the stolons' tips. Plants spend the dry summer months dormant, re-emerging after late-summer or autumn rains.
Flowers appear in the cooler months — autumn, winter, or spring — and are borne one to many on an upright, narrow but fleshy stalk. Each flower is small, roughly 4–5 mm in diameter, and green, pinkish, or purplish in colour, blending into the surrounding leaf litter and closely resembling a mosquito in silhouette. The sepals are longer than the petals and bear long, thin extensions; the dorsal sepal often forms a small hood over the column. The labellum is heart-shaped with a prominent callus and produces sweet nectar that attracts pollinating fungus gnats from the families Anisopodidae, Sciaridae, and Mycetophilidae. Fruit is a thin-walled, glabrous capsule containing numerous winged seeds.
The genus ranges across New Caledonia, Australia, and New Zealand. In Australia the greatest diversity is in New South Wales (9 species), with further species in Queensland, Victoria, and South Australia. The sole New Zealand representative, A. sinclairii, occurs on both the North and South Islands as well as on Raoul, Stewart, Chatham, and Three Kings Islands. Plants grow in small groups in forests on decaying litter or partially decayed logs, and sometimes form dense vegetative colonies in sheltered forest or heathland beneath shrubs and bracken.
Etymology
The genus name Acianthus is derived from the Greek ake or akis, meaning "a point" or "needle," combined with anthos, meaning "flower." This refers to the sharply pointed perianth segments and the acuminate tips of the floral parts that give the flowers their needle-like appearance.
Distribution
Acianthus is distributed across New Caledonia, Australia, and New Zealand. Australian diversity is centred in New South Wales (9 species), with additional species in Queensland (6), Victoria (3), and South Australia (2). The single New Zealand species, A. sinclairii, is found on both the North and South Islands and on several offshore islands including Raoul, Stewart, Chatham, and Three Kings Islands.
Ecology
Mosquito orchids grow in small groups on decaying forest litter or partially decayed logs, often forming dense vegetative colonies in sheltered forest and heathland, typically under shrubs and bracken. Pollination is carried out by fungus gnats from the families Anisopodidae, Sciaridae, and Mycetophilidae, which are attracted to a sweet nectar produced in a sunken area at the base of the labellum. Flowers on a single spike open sequentially over several days; the gnats' up-and-down probing motion brings them into contact with the viscidium and pollinia, which adhere to the insect and are transferred to the stigma of the next flower visited. Typically only a small proportion of plants in any colony bear flowers in a given season.
Taxonomy Notes
The genus was established by Robert Brown in 1810 in Prodromus Florae Novae Hollandiae, based on three species (A. fornicatus, A. exsertus, and A. caudatus); no type species was nominated at that time. Acianthus is placed in the family Orchidaceae, order Asparagales, and is regarded as closely related to Caladenia. Plants of the World Online treats several names (A. apprimus, A. borealis, A. collinus, A. exiguus) as synonyms of A. fornicatus, reducing the accepted species count. The Australian Plant Census (as of March 2025) and the New Zealand Plant Conservation Network (for A. sinclairii) are the current nomenclatural authorities for this genus.
